Communication systems are known to comprise a plurality of base stations that provide communication services to communication units located in corresponding service coverage areas of the base stations. One known communication system is the Global System for Mobile Communications (GSM) system. In the GSM system, a remote unit (e.g., a remote or portable radiotelephone) that desires to communicate, sends a channel request signal to a base station serving the coverage area in which the remote unit resides. Upon receiving the channel request signal, the serving base station allocates a communication channel for the communication and the remote unit begins communicating with a telephone network subscriber or another remote unit via the serving base station.
As the remote unit moves to the periphery of the serving base station's coverage area, and into the coverage area of neighboring base stations, a handover to a chosen base station is usually attempted to maintain call continuity. Under normal conditions, remote units are served by base stations that provide the strongest signal, and handovers are initiated when the server's received signal quality has fallen below a pre-defined threshold, or a neighbor's received signal becomes stronger than the server's received signal, after some appropriate averaging window and hysterisis margin. In the case that more than one neighbor base station is stronger than the serving base station, handover is ideally attempted to the strongest neighbor. Quantitatively, handoff is commonly attempted when PBGT(c,n).congruent.PL(c)-PL(n).gtoreq.H(c,n), where PL(c) is the path loss from the serving base station to the remote unit, PL(n) is the path loss from the neighbor base station to the remote unit, and H(c,n) is the handover margin between base station c and neighbor n, which in the ideal case is set to zero. (In practice the handover margin is set to a small, positive number by default to guard against ping-pong between mutual neighbor base stations).
In a live network, the traffic loads experienced by neighboring base stations tend to vary at different times of the day and commonly follows predictable patterns according to rush hours and centers of activities. Therefore, simply handing over to the neighbor base station with the best signal quality (i.e., having greatest power budget PBGT(c,n) above the corresponding handover margin) often times will not achieve the most system capacity since an opportunity to load share among less loaded neighbors may exist. Thus a need exists for a method and apparatus for determining optimal handover margins in a cellular communication system to achieve better load carrying capacity while maintaining acceptable signal quality.